1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a display device including a common voltage compensating circuit and a method of compensating a common voltage.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes a color filter substrate and an array substrate separated from each other by having a liquid crystal layer interposed there between, wherein the color filter substrate and the array substrate include a common electrode and a pixel electrode, respectively. When a voltage is supplied to the common electrode and the pixel electrode, an electric field is generated that changes the orientation of liquid crystal molecules of the liquid crystal layer due to optical anisotropy within the liquid crystal layer. Consequently, light transmittance characteristics of the liquid crystal layer is modulated and images are displayed by the LCD device.
Active matrix type LCD devices are commonly used because of their superiority in displaying moving images. Active matrix-type LCD devices include pixel regions disposed in a matrix form where a thin film transistor (TFT) is formed in the pixel region as a switching element.
An LCD device may be driven by a parity inversion method to prevent deterioration of a liquid crystal layer. In a parity inversion method, a polarity of a voltage applied to a pixel electrode is periodically inverted. A parity inversion method may be classified into a frame inversion method, a line inversion method and a dot inversion method. A dot inversion method, where a parity of a high level voltage of a data signal is periodically changed with a common voltage of a direct current (DC), is widely used because of its superiorities in display quality.
In an LCD device, a data signal of a data line, i.e., a pixel voltage is applied to a pixel electrode of an array substrate according to a state of a TFT, and a common voltage is applied to a common electrode of a color filter substrate. A liquid crystal layer between the pixel electrode and the common electrode is driven by a difference of the pixel voltage and common voltage to display images. While the liquid crystal layer is driven, however, a kickback voltage is generated due to a parasitic capacitance in the TFT. Accordingly, the pixel voltage deviates from the required value by the kickback voltage, and images having a required gray color are not displayed properly.
Moreover, when a data signal is changed from a first value corresponding to a black image to a second value corresponding to a white image, the common voltage deviates by a capacitance coupling due to the great difference between the first and second values of a data signal. In addition, when the LCD device is driven by a common voltage having a swing of a predetermined voltage difference, the common voltage deviation due to a capacitance coupling becomes greater. As a result, a horizontal cross-talk occurs and a display quality is deteriorated. The display quality of an LCD device is improved by a common voltage compensating circuit using feedback of a common voltage applied to the liquid crystal panel.
FIG. 1 is a schematic circuit diagram showing a common voltage compensating circuit according to the related art. In FIG. 1, a common voltage compensating circuit includes a voltage distributing unit 110 and a compensating unit 100. The voltage distributing unit 110 includes a resister “R” and a variable resistor “VR” in series between a source voltage “Vcc” and a ground. Accordingly, the source voltage “Vcc” is distributed to generate a distributed voltage. The compensating unit 100 includes an operational amplifier (OP AMP) “op” having an inverting input terminal (−), a non-inverting input terminal (+) and an output terminal. The distributed voltage from the voltage distributing unit 110 is input to the non-inverting input terminal (+) as a reference voltage and an output voltage of the OP AMP “op” is input to the inverting input terminal (−) by feedback. As a result, a first common voltage is generated using the reference voltage and a compensated common voltage is generated by feedback of the first common voltage.
In a common voltage compensating circuit according to the related art, however, the first common voltage input to the compensating unit 100 is not a resultant value used in a liquid crystal panel (not shown). After the first common voltage is applied to the liquid crystal panel, the first common voltage may deviate due to a state of the liquid crystal panel to be a second common voltage different from the first common voltage. Since the first common voltage not reflecting a state of the liquid crystal panel is used for compensation of a common voltage, the compensation of a common voltage is not exact and an improvement in display quality is limited. In addition, when the state of the liquid crystal panel changes during operation of the LCD device, the first common voltage should be amended manually.